[GHC] #10971: ExtendedDefaultRules defaults to [] for Foldable/Traversable

[GHC]

#10971: ExtendedDefaultRules defaults to [] for Foldable/Traversable
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        Reporter:  kanetw            |                Owner:  kanetw
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10963            |  Differential Rev(s):  phab:D1329
       Wiki Page:                    |
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Comment (by goldfire):

 Replying to [comment:6 simonpj]:
 > > Also, this implementation would seem to accept `default Either`, which
 is utterly meaningless, as `Either` has a non-defaultable kind. This is
 also utterly harmless, but it makes me suspicious of the design.
 >
 > It would not be utterly meaningless if we added some new type class of
 kind `(* -> * -> *) -> Constraint`, and wanted defaulting to work on it.
 >
 > Simon

 That's true. But we have no such classes with defaulting yet, and it would
 take a change to the compiler to make it so.

 But, I've heard a loud enough chorus of voices asking for just one list
 (including some on IRC). And I think KaneTW's argument in comment:4 is
 convincing. I stand down.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10971#comment:7>
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